This invention relates to radiation systems, and more particularly to a radiation projection printer using actinic lithography for providing a uniform field of irradiation for making contact or proximity prints on wafers and masks.
In the field of optical exposure systems for exposing photoresist material deposited on wafers in the production of integrated circuits and the like, the mask is positioned in close proximity to the wafer at the lens exposure plane which is characterized by a relatively uniform field. By so positioning the mask, one is utilizing the principle used in systems that are at times called "reverse projection systems" or "shadow projection systems" using refractive optics. It should be understood that the uniform field of the irradiance source is effectively projected through the mask and thence to the proximate wafer, whereas in a conventional projection system the lens projects the image of the object mask onto a wafer that is remotely positioned from the object mask. A reverse projection system which, as just indicated, may be also termed a shadow projection system, is described in my U.S. Pat. No. 3,860,335 entitled "OPTICAL SYSTEM" issued on Jan. 14, 1975.
Reference is made to an article by D. A. Doane, entitled "Optical Lithography in the 1-.mu.m Limit" published in Solid State Technology, August 1980, pp. 101-114 for a description of the state of the art and particularly the problems confronting those in optical lithography.
While the shadow projection system of the type described in my aforementioned patent performs adequately for systems in which the smallest linewidths of the wafers are in the order of seven micrometers, as the art advances, there is a need for still finer resolution of linewidths on wafers and masks on the order of 5 micrometers and, more importantly, on the order of 1-3 micrometers. Such a resolution could not be achieved heretofore by the use of various types of lamp sources such as mercury arc lamps and other similar lamp irradiance sources that do not provide uniform light distribution over a broad spectral region. Helical xenon flash tubes are known to develop a broad spectrum of radiation in the actinic spectral region. Such polychromatic radiation it now appears is extremely useful in developing very fine resolution lines. However, in order to achieve such fine resolution, there is an additional problem of collecting a broader spectra from the source to project onto the mask and to do so with a lens systems that has a higher numerical aperture (N.A.) with a minimum of diffraction effects and spherical aberration.
There is a need, therefore, for a system of the shadow projection system type to provide improved resolution and uniformity in the projected image.